Your team's health should never be so low as you don't have enough time to get out of the rings area without a member dying, it only takes about 2-3s at most. I have healed this on my sorc which isn't a very mobile healer without any problems. It's vital you get out of the ring otherwise chances are you won't be throwing out any heals whatsoever since you'll be dead.
This missiles can hit my sage for 25k, everyone except the tank shall be kept almost full health and anybody with less health can be instant killed. Sages have no ability they could cast to heal that much while moving. Furthermore this tend to lag. Many time, we are such as 10m away from the circle when getting the hit. However I found a trick; I'm waiting someone else is targeted to start casting my more powerful but slower heals, otherwise I run and cast quick heals. I also ran it with an operative and had no such issues, the strongest heals they have can be cast while running.